Tbh I do commend AT for making headphones with solid build quality that can take a solid beating while also being foldable in the age of planned obsolescence. A guy once told me people who still use the m40x are stuck in the 2010s, then my response was that the reason the m40x is still relevant today is because they last so freaking long that people who bought it on release back in 2010s are still using them today lmao, and I see why people swear by them.
I think the sound card functions as a dac already, not sure if it’s an amp, but the pc38x has low impedance and high sensitivity which makes it easy to drive and probably doesn’t need a dedicated external amp. I’m confused when you say the audio is too low but when you increase the volume knob, it becomes too loud so driving isn’t an issue, just find a healthy middle ground by slowly adjusting both while listening to some songs.
Regarding sound quality itself, this becomes a lot more subjective. Try to set your sbx profile to something neutral or turned off first, then go from there. In general as long as there’s no obvious crackling, distorted sound or channel imbalance where one side is louder than the other side (unless the song itself does that), that means it’s working fine. You would have to describe in better detail what makes you think the “audio sound bad”. Does it lack bass? Voices sound muffled? High end notes too strong or too weak? All of these are less of the headphones being broken and more of headphones having their own sound (frequency response). PC38x is rather neutral with the bass mostly flat and rolled off, while a lot of gaming headsets have boosted bass so a person who likes bass may dislike the pc38x because of it.
Next is mostly subjective, what kind of songs do you listen to? People who like hiphop and rnb usually enjoy bass while other genres like classical may not care as much about it. Individual preference matters a lot here. Here you can start experimenting with your Sbx or eq software, just grab some of your favorite songs and A-B listen to them with certain profiles on or off. EQ isn’t inherently bad, sometimes you have a headphone you enjoy but just wish it had more oomph in some places so you start using EQ. If you don’t want to use EQ that’s also fine, try the headphones with the eq/sbx settings at neutral or off for a week or 2 and you might get used to it. I started using an open back this year and going back to my old headsets, I felt they are now too bassy when previously I was fine with them.